Erasable Programmable Read-Only Memory (EPROM) is one known form of non-volatile memory. Electrically Erasable Programmable Read-Only Memory (EEPROM or E2PROM) is a related type of non-volatile memory. As used herein, references to EPROM include both EPROM and EEPROM. EPROMs and EEPROMs are generally realized as arrays of conventional floating-gate transistors. The conventional floating gate or “double gate” transistor is a transistor arrangement that is commonly used for non-volatile storage such as flash, EPROM and EEPROM memory. Floating-gate metal-oxide semiconductor field-effect transistors (MOSFETs) comprise a conventional MOSFET and one or more capacitors stacked thereon that are used to couple control voltages to the floating gate. A dielectric, generally an oxide, surrounds the floating gate, so that any charge trapped on the floating gate generally remains there. The charge stored on the floating gate can be modified by applying voltages to the source, drain, body and control gate terminals of the MOSFET, such that the electric fields generated result in phenomena such as Fowler-Nordheim tunneling and hot carrier injection. The extra gate in the conventional double gate structure represents several additional process steps as compared to a conventional MOSFET process.
There is frequently a requirement to incorporate memory in VLSI chips. However, with shrinking device geometries, devices including memory devices such as EPROMs have begun to experience undesirable short channel effects, especially “off-state” leakage current, which increases the idle power required. What is needed is a low leakage EPROM design that is also area efficient, and adds little or no extra processing steps.